Front projection screen made from a transparent material

ABSTRACT

A front projection screen is disclosed which is made from a transparent material. Prism like elements are formed on the back side thereof so that light incident upon the screen from a preferred direction will be totally reflected by the screen while light incident upon the screen from other directions will pass therethrough. Sides of the prism like elements are curved to provide dispersion in the horizontal plane while the front surface of the projection screen has segments of cylindrical columns formed thereon perpendicular to the prism like elements to provide an angular spread in the vertical plane.

United States Patent 1191 Brown Jan. 1, 1974 FRONT PROJECTION SCREENMADE 2,351,034 6/1944 Gabor 350/128 x FROM A TRANSPARENT MATERIAL2,551,954 5/1951 Lehman 350/167 [75] Inventor: John Brown New HopePrimary Examiner-Samuel S. Matthews [73] Assignee: Qantix Corporation,Flemington, Assistant Examiner-E Bel'o N Att0rneyLawrence l. Lerner eta].

[22] Filed: Nov. 30, 1972 [57] ABSTRACT [21] Appl- NO-I 310,882 A frontprojection screen is disclosed which is made from a transparentmaterial. Prism like elements are 52 us. c1 350/129, 350/128, 350/167fmmed the back me there f l f [51] Int CL Gosh 21/60 upon the screenfrom a preferred dlrection w1ll be to- I tan reflected y the screenWhile light incident p [58] Fleld of Search 350/128, 167, 106, y

350/109 127 129 the screen from other directions will pass therethrough.Sides of the prism like elements are curved to Eliliiifiiiiifi31350153221221, ZELZZZQJL? UNITED STATES PATENTS cylindrical columnsformed thereon perpendicular to g l z the prism like elements to providean angular spread 0 p 595,273 12/1897 Soper l 350/167 m the vemca] plane2,180,093 1 1/1939 Personis 350/106 5 Claims, 3 Drawing Figures 1 I I II I I I I I .L'I m' E 'mrfimi/i' w in /Na FRONT PROJECTION SCREEN MADEFROM A TRANSPARENT MATERIAL FIELD OF THE INVENTION This inventionrelates to projection screens and particularly front projection screens.

BACKGROUND OF THE INVENTION Projection screens are well known in the artwhich are employed for picture display. Most screens as they aretraditionally known have a material on the surface thereof for thepurpose of dispersing light in a multitude of directions which isincident upon the screen from the projector.

In situations where the efficiency of the screen is important, attemptshave been made to direct light incident upon the screen from theprojector into a predetermined viewing area. For example in my priorpatent, Ser. No. 178,990 which was filed on Sept. 9, 1971 and isentitled Lenticular Projection Screen now US. Pat. No. 3712708, alenticular projection screen is disclosed which consists of a pluralityof overlapping spherical concave lenses formed in reflective material.The lenses and the screen are constructed so that the projected lightrays are reflected to predetermined viewing areas in front of thescreen.

When screens are made of a reflective material it is of course obviousthat reflections from sources other than the projector become a problem.In the past people have overcome this problem by using the screen in adark room. Attempts have been made to use such screens in lighted areasby putting light absorbing elements in predetermined locations withrespect to the reflective material in the screen. These attempts havebeen somewhat succesful but have added additional annoyance to the useof the screen and/or additional cost and encumberance in the manufactureand use of the screen. Notwithstanding these efforts to overcome theproblems of unwanted reflections, difficulties are still encounteredwhen using these screens.

Even in a dark movie theater, people open the door to come in and go outand ushers use flashlights which create difficulty with reflectionstherefrom. The use of light absorbing material while having a limitedbeneficial affect still does not completely overcome the problem ofunwanted reflections.

On Dec. I4, 1926, US. Pat. No. 1,610,423 issued to A. J. Cawley whichwas entitled Daylight Projecting System. In FIG. 4 of this patent adaylight projection screen is disclosed in which a transparent materialis shaped to have prism like ridges on the back thereof for the purposeof reflecting light which is incident thereon from a predeterminedangle. Cawley teaches that the critical angle of the material should beas close to 45 as possible and that the angle of the sides of the prismlike portions should be inclined with respect to the plane of thematerial at 45. The reason for this is to insure that only lightincident upon the screen from a single direction will be reflected backtowards the audience. All other light incident upon the screen willstrike one side of the prism or the other at an angle less than 45 withrespect to a normal to the surface thereof and pass therethrough ratherthan be reflected.

The Cawley system is an interesting curiousity but is of no practicalsignificance since all of the light is focused back in a singledirection so that the audience must-be located directly in front of thescreen. If the screen is moved by an angle of say 1 degree with respectto the incoming projected light, the image reflected back to theaudience will be lost because the largest portion of the light incidentupon the screen from the projector will pass therethrough rather thanbeing reflected back. Further if the audience is not sitting at theprecise angular relationship with respect to the screen, they will againsee very little because the screen of Cawley will project light backparallel to the rays incident thereof and provide no dispersive angularaffect of reflection.

Therefore, it is an object of this invention to provide a new andimproved front projection screen.

It is a further object of this invention to provide a front projectionscreen which efflciently uses the light provided by the projector, doesnot reflect undesirable images to the audience and is viewable frompositions other than directly in front of the screen.

It is still another object of this invention to provide a frontprojection screen which may be employed in daylight with a minimum ofreflection without the use of light absorbing members affixed thereto.

BRIEF DESCRIPTION OF THE INVENTION With these and other objects in viewthe present invention contemplates a front projection screen made from asheet of light transmitting material having a predetermined criticalangle of internal reflection less than 45; the sheet has a front surfaceand a back surface; the back surface having formed thereon a pluralityof parallel ridges; each of the ridges have curved sides terminating ata peak; the curved sides extend away from the sheet at an angle equal toor greater than 45; the angle of the sides with respect to the sheetcontinuously decrease as the curved sides extend towards the peak; theangle of the sides being greater than or equal to the predeterminedcritical angle of internal reflection at the peak but less than 45.

In the preferred embodiment of the invention the front surface hasformed thereon a plurality of parallel ridges each forming a section ofa cylinder. The ridges on the front surface are disposed perpendicularlyto the ridges on the back surface for the purpose of giving dispersionof the light in the vertical plane. In the preferred embodiment thereare at least 500 ridges on the backsurface of the screen to providesufficient resolution to project a normal television picture thereonwithout degrading the image thereof.

Further, in the preferred embodiment the thickness of the screen is madegreater than the focal length of the cylindrical sections on the frontthereof but less than one and one half times the focal length to providea structure which not only performs the desired function but ismechanically able to be relatively selfsupporting.

DESCRIPTION OF THE DRAWINGS For a more complete understanding of theinvention, reference should be made to the following detaileddescription and drawings in which:

FIG. 1 is a schematic prospective view of a front projection systememploying a screen constructed in accordance with the teachings of thisinvention;

FIG. 2 is a front prospective view of a section of the screen shown inFIG. 1; and

FIG. 3 is a back prospective view of the screen shown in FIG. 1..

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1 we see afront projection systcm employing a screen constructed in accordancewith the teachings of this invention. A projector 10 is mounted a fixeddistance from a screen 11 which is formed as a section of a spherehaving a radius of curvature equal to the distance between the screen 11and the projector 10. It is of course understood that as the distancebetween the projector 10 and the screen 11 becomes greater, the need forthe precise curvature of the screen 1 1 becomes less important so thatas the distance becomes substantial the screen 11 may in fact be flat.The reason for the curvature of the screen 11 is to insure that the raysof light emanating from the projector l incident upon the screen 11strikes a front surface of the screen 12 normal thereto at each pointthereof.

Referring now to FIG. 2 we see the details of a small section of thescreen 11. It should be understood that the screen is uniform throughoutso that the section shown in FIG. 2 is representative of the makeup ofscreen 1 l.

The screen 11 is made from a transparent material which has a criticalangle of internal reflection less than 45 degrees. A critical angle ofinternal reflection is defined as the angle at which light must strike asurface of a material when passing from the inside to the outsidethereof to a second predetermined medium so as to be totally reflectedrather than passing to the second medium. The angle of incidence asreferred to above is measured between the ray of light and a line normalto the surface of interest at the point of incidence. The critical angleof internal reflection is the angle which defines the limit of totalreflection so that any incident light beam which has an angle ofincidence greater than the critical angle will be totally reflected.

As can be seen when looking at FIGS. 2 and 3 the back surface of thescreen 11 has a plurality of ridges thereon which form prism likeelements. The ridges 13 are parallel to each other and have curved sides14 and 15. Each of the curved sides 14 and 15 meet at a peak thereofwhich appears in FIG. 3 as a line.

The angle of the sides 14 and 15 with respect to a plane extendingthrough the screen 11 at the point of interest is equal to or greaterthan 45. As the sides 14 and 15 extend away from the screen 11 towardsthe intersection thereof, the angle with respect to the aforementionedplane decreases until the intersection is reached. At the intersectionor peak, the angle of the side 14 and/or 15 with respect to theaforementioned plane is less than 45 but greater than or equal to thecritical angle of internal reflection of the material forming the screen11. The curvature of the sides 14 and 15 as above described is the keyto the functioning of the instant screen. In the first place, withoutregard to any other considerations, the curvature of the sides 14 and 15allow a certain amount of leeway in the angular relationship between theprojector and the mounting of the screen 11. As will be appreciated if apredetermined angle were necessary with a tolerance of 1 or less, theslightest movement of the projector 10 or the screen 11 would render thesystem less functionable and therefore would detract from it as acommercial item. By curving the sides of the screen 14 and by, forexample 6 to 10, a considerably greater range of angular tolerance isbuilt in without greatly adding to the directions from which ambientlight can be reflected back to the audience.

Further advantages accrue from the curvature of the sides 14 and 15which render the screen 11 of the instant invention a commerciallyusable item. By curving the sides, the direction in which light incidentupon the screen 11 is transmitted back towards the audience isdetermined by the portion of the side 14 or 15 upon which it strikes.Therefore if the sides 14 and 15 were straight, all of the lightincident thereon would either be reflected directly back towards theprojector or if at an improper angle of incidence would passtherethrough. Under the teaching of the instant invention, the lightincident upon the screen 11 at a predetermined angle will be reflectedback towards the audience at a plurality or range of angles which can becontrolled by the degree of curvature of the sides 14 and 15 to providea predetermined viewing area greater than merely directly in front ofthe screen. While this curvature does increase the directions from whichlight will be reflected back, the small angular range of viewing areasnecessary for display purposes renders the compromise a practicalcommercial one.

A light ray emanating from the projector 10 and striking the screen 11so as to hit the surface 14 or 15 close to a trough will be hitting asurface of approximately 45. Such a light ray will be totally reflectedand passed across to an opposite side thereof and is reflected backtoward the projector parallel to the incident light ray. It should benoted at this point that a reversal or inversion takes place in smallsegments of the picture as the light rays pass through the prism likeridges 13.

It will be appreciated that as the position of incidence of a light rayvaries along a side 14 or 15 of a ridge 13, the angle at which the lightray will strike the back surface of the screen 11 will be altered sinceall incident light rays are parallel to each other. Therefore as theposition moves from the 45 at the troughs of the ridges 13 towards thelessening angles at the peaks thereof, the direction in which the lightray is reflected back towards the audience is varied. It will beappreciated that since there are two reflections and each time areflection occurs, the angle of deviation from 45 is multiplied by twoin terms of direction of reflection, a six degree curvature in the sides14 and 15 of the ridges 13 will result in a dispersion of 24 on eachside of the normal to the screen. It will of course be appreciated thatadditional dispersion occurs due to refraction as the light ray passesback out the front face of the screen 11 so that the small curvature atthe back thereof provides a substantial degree of dispersion of theprojected light. In contradistinction to this if in fact the sides werestraight, not only would the dispersion caused by the curvature of thesides 14 and 15 not occur, but no refraction would occur since the lightentering the front face of the screen 1 1 normal thereto would exitnormal thereto and be undisturbed.

At this point it is important to understand that the curvature mustbegin at an angle of 45 or greater since no light would be reflectedback towards the audience if there were not a 45 angle. Therefore, the45 angle provides the beginning point of total reflection. An angle ofgreater than 45 will not reflect at all in the present configurationsince one of the sides 14 or 15 would allow the light ray to passcompletely therethrough or would reflect the ray elsewhere. It isthought that it is better to allow a small amount of light to be lostrather than have a dead spot in the center of an audience. 1n thepreferred embodiment, the critical angle of the material making up thescreen 11 is less than 45 by an amount equal to the degree of curvaturedesired in the sides 14 and 15. Thus if a 6 curvature were desired, amaterial having a critical angle of 39 would be employed so that thesides 14 and 15 would vary in angle of curvature from slightly greaterthan 45 to cover the center portion of the audience to 39 at the peak.As a result of such an arrangement, all of the projected light incidentupon the screen would be reflected back towards the desired portions ofan audience array while the maximum angle of directions from whichambient incident light would be reflected is minimized.

Several materials exist which have critical'angles in internalreflection with respect to air with approximately 39. For example,polystyrene has a critical angle under such circumstances of 38.93 whilepoly (N-2,phenethyl) methacrylamide has a critical angle of 39.01 andpoly (o-tolyl) methacrylate has a critical angle of 39.54. Listed beloware additional materials which are suitable for use as the materialmaking up the screen 11 with their critical angle with respect to air at20 to centigrade listed adjacent thereto:

MATERIAL CRITICAL ANGLE polymethylacrylate 42.53 polyethylacrylate 42.92polytutylaerylate 43.01 polyethoxyethylacrylate 42.83 poly (2methoxyethyl) acrylate 43.12 poly (2 bromo sec. butyl) acrylate 40.43poly (2 bromo phenyl) acrylate 38.34 poly (2 chloromethyl) acrylate41.23 polyacrylonitrile 41.47 polymethylmethacrylate 42.16polyethylmethaerylate 42.33 poly butyl methacrylate 42.4 poly (t-butyl)methacrylate 43.09 polycyclohexyl methacrylate 41.59 poly(Z-hydroxyethyl) methacrylate 41.41 poly (2-phenoxyethyl) methacrylate39.96 poly phenylmethacrylate I 34.82 poly (o-chloro) styrene 38.4 poly(2,6 dichloro) styrene 37.99 poly (O-methoxy) styrene 38.87 polyacetal 4I .47 poly (n-benzyl) methacrylamide 38.78 poly (N-butyl) methacrylamide41.36 polyvinyl chloride 40.53 polyvinyl fluoride 38.68 polyvinylidenechloride 38.68 polyvinyl acetate 42.97 polyvinyl carbazole 36.36polyvinyl isobutyl ether 43.58 polyvinyl alcohol 41.81 poly (n-vinyl)phthalimide 38.13 polyallyl phthalate 41.21 polyester-Styrene 40.5 polycarbonates (bisphenol) 39.12 zinc crown glass 41.24 higher dispersioncrown glass 41.14 light flint glass 39.41 heavy flint glass 37.31heaviest flint glass 31.94

Looking at the front surface of the screen 12, it will be noted that aplurality of ridges 16 are formed thereon which are disposedperpendicularly to the ridges 13 formed on the back surface of thescreen. It will be noted that the ridges 16 are formed from arcuateportions of cylinders. In this particular instance, each ridge 16 formsthe surface of of a cylinder. These ridges 16 act as optical elementsfor operating on the incoming light from the projector 10 for purposesof creating dispersion of the light to a predetermined range of areas infront of the screen 11 in the vertical field.

It should be appreciated that the ridges l6 operate in conjunction withthe reflecting surfaces 14 and 15 to provide the dispersion in thevertical plane. Therefore, the position of the surfaces 14 and 15 withrespect to the ridges 16 (the thickness of the'screen l1) determines thedegree of dispersion produced by the ridges 16. Therefore if the backsurface of the screen 11 made up of the surfaces 14 and 15 wereconsidered flat and positioned at the focal point of each of the ridges15, there would be no vertical dispersion of light. Rather, all of thelight would be focused to come out parallel to the incident light. Byvarying the thickness it will be appreciated thata vertical field willbe developed as the thickness of the screen is made less than the focallength of the ridges 16. The angle of dispersion and therefore thevertical field of the screen is increased until the thickness approacheszero.

While in some circumstances a compromise can be arranged between maximumdispersion at zero thickness and thickness equal to the focal length,the thickness of the screen will still be such that additionalmechanical support would be necessary for the screen 1 1 in order tosupport the weight of the screen. It should be noted that any mechanicalsupport cannot touch the screen 11 in its active area since such supportwill alter the optical characteristics of the screen and thereforeeffect its functioning.

It has been found that by making the screen 11 thicker than the focallength of the ridges 16, light will exit the front surface of the screen11 from a different ridge 16 than it entered. It has further beendiscovered that if the thickness of the screen is just slightly greaterthan the focal length of the cylinders 16, light will exit the incidentridge 16 parallel to the incident ray and the adjacent ridges 16 at themaximum angle of dispersion so that in effect, three viewing positionswill result in the vertical plane. Further, in accordance with thisinvention it has been discovered that as the thickness of the screen isincreased from the focal length of the ridges 16, the spread of verticaldispersion in the incident ridge 16 will also increase from the maximumtowards the direction of incident light in the incident ridge 16. It hasbeen found that a preferred thickness exists in which an entire range ofvertical dispersion can be covered with a portion thereof emanating froman incident ridge 16 and the remaining portion of the range emanatingfrom an adjacent ridge 16. It should be understood that each ridge 16serves as an incident ridge for light incident thereon while alsoserving as adjacent ridges 16 for light reflected from ridges adjacentthereto.

This might seem confusing and might seem to provide resolution problemsbut it must be remembered that an individual sitting in aparticular-viewing relationship to the screen 16 will only see lightemanating in that direction thereby providing a slight shift in apicture but will not provide resolution problems. lnterestingly thoughit has been found that if the thickness were increased beyond one andone half focal lengths light will spill into a further adjacent ridge16. Light from a single incident beam would then be exiting from morethan one ridge 16 resulting in resolution difficulties.

As can be appreciated the processing of the light by the screen 11 asdescribed above, results in increased intensity of light inpredetermined areas in front of the screen by concentrating incidentlight into the desired position. This is done in a piece-wise manner bybreaking up the picture into columns and rows by the ridges 13 and 16and focusing it back out where appropriate. It can be appreciated, ofcourse, that the size of the ridges 13 and 16 must be close together soas not to create resolution difficulties. As mentioned above, the ridges13 cause a reversal of a small portion of the picture so that distinctresolution problems would occur if they were sufficiently large.Therefore, the number of ridges 13 and 16 are made large compared withthe resolution desired. Therefore, in order to display a picture such asa television picture having a resolution of over 500 lines, it isdesirable that over 500 ridges 13 and 16 be provided on a screen forprojection thereof.

It should be understood that while this invention has been describedwith respect to a specific embodiment thereof, numerous others maybecome obvious to those of ordinary skill in the art in light of thisdisclosure.

What is claimed is:

1. A front projection screen made from a sheet of light transmittingmaterial having a predetermined critical angle of internal reflectionless than 45; said sheet having a front surface and a back surface; saidscreen being characterized by:

said back surface having formed thereon a plurality of parallel ridges;each of said ridges having curved sides terminating at a peak; saidcurved sides extending away from said sheet at an angle equal to orgreater than 45 immediately adjacent to said sheet; the angle between aline tangent to said sides and said sheet continuously decreasing assaid curved sides extend towards said peak; the angle between a linetangent to said sides and said sheet being greater than or equal to saidpredetermined critical angle of internal reflection at said peaks butless than 45.

2. The front projection screen as defined in claim 1 characterizedfurther by:

said front surface having formed thereon a plurality of parallel ridgeseach forming a section of a cylinder; said ridges on said front surfacebeing disposed perpendicularly to said ridges on said back surface.

3. The front projection screen as defined in claim 2 in which said backsurface has a minimum of 500 ridges thereon.

4. The front projection screen as defined in claim 3 in which said frontsurface has a minimum of 500 ridges thereon.

5. The front projection screen as defined in claim 3 in which theparallel ridges which form sections of cylinders each have therefor afocal length and the distance from said front surface to said backsurface is between one and one and one half times said focal length.

1. A front projection screen made from a sheet of light transmittingmaterial having a predetermined critical angle of internal reflectionless than 45*; said sheet having a front surface and a back surface;said screen being characterized by: said back surface having formedthereon a plurality of parallel ridges; each of said ridges havingcurved sides terminating at a peak; said curved sides extending awayfrom said sheet at an angle equal to or greater than 45* immediatelyadjacent to said sheet; the angle between a line tangent to said sidesand said sheet continuously decreasing as said curved sides extendtowards said peak; the angle between a line tangent to said sides andsaid sheet being greater than or equal to said predetermined criticalangle of internal reflection at said peaks but less than 45*.
 2. Thefront projection screen as defined in claim 1 characterized further by:said front surface having formed thereon a plurality of parallel ridgeseach forming a section of a cylinder; said ridges on said front surfacebeing disposed perpendicularly to said ridges on said back surface. 3.The front projection screen as defined in claim 2 in which said backsurface has a minimum of 500 ridges thereon.
 4. The front projectionscreen as defined in claim 3 in which said front surface has a minimumof 500 ridges thereon.
 5. The front projection screen as defined inclaim 3 in which the parallel ridges which form sections of cylinderseach have therefor a focal length and the distance from said frontsurface to said back surface is between one and one and one half timessaid focal length.